MD Physics

[green]

I read reducing the target size by a half reduces the sensitivity to one eight. Did a test this morning, appears to be true if I double the thickness to keep the TC the same. The 8x8mm 1 layer projects a higher amplitude than the 8x8mm 2 layer at 3usec. Test is with a PI, would a VLF show different? What am I missing or doing wrong?

[kt315]

Eric must sit to write a book INSIDE THE METAL DETECTOR 4ALL


Re: Question on coils

Posted by: Eric Foster
Date: May 18, 2005


Here are the curves I have used for many years. The range reaches a maximum when it is equal to the radius of the coil. Coils larger or smaller than this optimum will result in less range. To show how this works, along the bottom axis you see coil diameter, which is obviously 2 x the radius. So for an 11in coil, if we go up the vertical scale to A, we have 5.5in. Also note the diagonal line and the series of ever increasing semicircles. Everything to the left of this line shows increasing detection range up to the maximum where it intersects the line, then decreasing range to the right, where the semicircles are shown dashed.

If a certain metal object is just detected at 5.5in with the 11in coil, then going larger in coil size will cause a reduction (going down the dashed side), and going smaller in coil size will have a similar effect. Initially, it won’t be much, i.e. going from an 11in to an 8in coil will only make 0.5in difference but below 4in diameter, the range will drop rapidly.
Now, suppose with the 11in coil, you can detect an object at about 12.5in (B on the vertical scale. This indicates that the coil is not an optimum size for that particular object. If we carry on up the curve (direction of arrow) we can see that by using a 20in coil, we could gain another 2.5in (C). The curve peaks at 15in with a 30in coil. But the extra inch gained hardly makes such an unwieldy coil worth while.

Other factors come into play of course. The curves assume that the number of turns and the coil current is the same in all cases; which it isn’t necessarily. For the same inductance value, a smaller coil has more turns, which counteracts to some degree the loss in range. Also a smaller coil will pick up less electromagnetic noise, earth’s field noise and ground effect, which make for a smoother threshold.

The end result is, that with a small nugget that can be detected at between 5 and 7in with the 11in coil, so that it is on the top part of the curve, an 8in coil may well give a similar range. That is not to say that smaller coils do not have other advantages. Small coils and probes are very useful in rocky areas or searching in undergrowth. They have less drag too for water hunting, and less pickup from mineralised soil or conductive sea water plus better signal separation on close or multiple objects.

One other point regarding PI, is that the small object sensitivity is largely determined by the sample pulse delay. If an object is so small, or thin, or made of high grade stainless steel, such that all the signal has decayed before sampling takes place, it would not matter how small a coil you made, it would never be picked up.

Eric.

[kt315]

Test is with a PI, would a VLF show different?
---
sure NO. AC electromagnetic field has ONE NATURE either its PI or VLF.

[Carl-NC]

I read reducing the target size by a half reduces the sensitivity to one eight. Did a test this morning, appears to be true if I double the thickness to keep the TC the same. The 8x8mm 1 layer projects a higher amplitude than the 8x8mm 2 layer at 3usec. Test is with a PI, would a VLF show different? What am I missing or doing wrong?

Not sure what you are asking... are you wondering why the 8x8x1 has a stronger response than the 8x8x2? Also, your 16x16 target dimension is 2x, but the area is 4x. Not sure if this is intended.

[green]

I read reducing the target size by a half reduces the sensitivity to one eight. Did a test this morning, appears to be true if I double the thickness to keep the TC the same. The 8x8mm 1 layer projects a higher amplitude than the 8x8mm 2 layer at 3usec. Test is with a PI, would a VLF show different? What am I missing or doing wrong?

One of the reasons for trying to understand the physics. I have a gold nugget, 4 grains(TC=3.1usec). I read about detecting lot smaller nuggets. Wondering how small is detectable and what the nugget TC might be. Thought I would make a target with 1usec TC. Some data, think it is correct. Signal for 2x2mm 8 layer target less than needed but looks like it might parallel the other targets. Assuming 1x1mm 16 layer would parallel with 1/8 the signal of 2x2mm 8 layer target. Maybe 1usec TC is to low? Loose 90% of signal if delay is increased by 2.3usec for 1usec TC target.

[green]

Not sure what you are asking... are you wondering why the 8x8x1 has a stronger response than the 8x8x2? Also, your 16x16 target dimension is 2x, but the area is 4x. Not sure if this is intended.

Trying to understand(Coil Sensitivity) page 247 ITMD. Where as sensitivity is approximately equal to the cube of target diameter. Statement doesn't mention thickness. Just trying to understand what and how much effects target strength. Maybe I'm looking at it wrong, wouldn't be the first time.

When I reduced the width in half and doubled the thickness to keep the TC the same the signal reduced to 1/8(cubed). The smaller single layer target has a larger signal if sampled fast enough, not reduced signal.

[green]

Wondering if a 1x1x.26mm target made of aluminum is detectable in air(targets reply #5). If yes, VLF or PI or both, coil size? Detection distance?

[Carl-NC]

I don't have an exact answer, but I expect "sensitivity is approximately equal to the cube of the target diameter" is valid for higher conductive coins where skin effect is not a major factor. For low conductors, skin effect does play a bigger role and has to be accounted for.

The reason you see a stronger signal with the small single layer target might be due to the exact configuration of your circuit. The step response of an eddy target is generally t*e^(-t/tau) which shows a characteristic peak followed by the exponential decay. Sampling exactly on the peak gives highest sensitivity, but only for that tau. Many PI designs are more sensitive to low conductors than high conductors; they almost always detect nickels deeper than quarters, even though quarters are bigger. The preamp design also plays a role, especially the bandwidth which alters the step response. Play around with bandwidth, sample delay, and sample width and you'll see this effect.

In comparing PI & VLF, it's all about sample window (PI) and frequency (VLF). In PI it's difficult to get the sample delay low enough to see sub-grain nuggets. In VLF it's easier to get the frequency high enough to see sub-grain nuggets.

[pustareka]

[Skippy]

Quote Carl:" I expect "sensitivity is approximately equal to the cube of the target diameter" is valid for higher conductive coins where skin effect is not a major factor. For low conductors, skin effect does play a bigger role and has to be accounted for.

I would say this is incorrect. High conductivity materials like copper, silver are the most affected by skin effects at high frequencies. Low conductor metals like cupro-nickel, lead, maintain greater skin depths as frequency increases.
This came up in one of the other target modelling threads Green & myself have contributed to.
This Wiki page explains it mathematically:
https://en.wikipedia.org/wiki/Skin_effect

Though I should add that bits of aluminium kitchen foil are so thin that skin depth is irrelevant, at VLF MD operating frequencies, skin depth is always going to be greater than the foil thickness.

[green]

I don't have an exact answer, but I expect "sensitivity is approximately equal to the cube of the target diameter" is valid for higher conductive coins where skin effect is not a major factor. For low conductors, skin effect does play a bigger role and has to be accounted for.

The reason you see a stronger signal with the small single layer target might be due to the exact configuration of your circuit. The step response of an eddy target is generally t*e^(-t/tau) which shows a characteristic peak followed by the exponential decay. Sampling exactly on the peak gives highest sensitivity, but only for that tau. Many PI designs are more sensitive to low conductors than high conductors; they almost always detect nickels deeper than quarters, even though quarters are bigger. The preamp design also plays a role, especially the bandwidth which alters the step response. Play around with bandwidth, sample delay, and sample width and you'll see this effect.

In comparing PI & VLF, it's all about sample window (PI) and frequency (VLF). In PI it's difficult to get the sample delay low enough to see sub-grain nuggets. In VLF it's easier to get the frequency high enough to see sub-grain nuggets.

Thanks for the reply. Is there a best VLF Tx frequency for a 1usec TC target? Cut some targets from aluminum foil and copper tubing and charted data. Thicker targets are closer to diameter cubed during normal PI sample times. Would the change in TC effect the cubed change with diameter with a VLF?

[Skippy]

Quote:"Is there a best VLF Tx frequency for a 1usec TC target?"

A 1 microsec TC target would be equivalent to a 159kHz corner frequency. So in principle, a detector operating in this sort of range should be best. However, I assume a 1 usec target is the smallest target you're realistically trying to find, you would rather hope to find ones with 5 or 10 usec TC ? So a typical VLF 'gold' machine operating at 35 - 70 kHz would probably be a realistic choice.

[green]

Back home the Vallon coil would not detect a 1in square of aluminium baking foil but would detect a 1.5in square at 8in. My 11in coil would only detect the 1.5in square at 3in. The detection of low conductors seems to fall off very sharply and seems more dependant on coil size than I have noticed before, with other PI detectors. It will however, easily detect a 4mm diameter phosphor bronze ball and a 0.3gm gold nugget with the Vallon coil but the 11in coil gives less range on those targets.
from http://www.geotech1.com/forums/showt...768#post217768

The 1 inch square of aluminum foil has a TC more than 1.5 times the 1usec charted targets with a signal strength probably more than 10000 times the 1x1x.26mm target. I was surprised the vallon wouldn't detect 1x1 inch aluminum foil. Don't have any gold near me to detect. Just trying to understand the variables. Think target TC is proportional to width*thickness. Think Rx amplitude is proportional to width*length. Reducing width in half reduces length in half, 1/4 the signal. Needed to double thickness to keep TC the same so I'm guessing doubling thickness reduces Rx signal in half for I/8 the signal. The 1/8 seems to be a ballpark number if target can be sampled at the right time.

The 16x16mmx1 layer target(TC=1usec)is chartable, so easy to detect. The 2x2mmx8layer target(TC=1usec)almost chartable, maybe detectable with 200mm coil. Don't know if I could detect the 1x1mmx16 layer, .26mm thick(TC=1usec)with a 200mm coil, wondered if anyone could detect it. Need to finish the detector to try. A piece cut from an aluminum can side 5x5mmx1 layer has a 1.1usec TC, should have a Rx signal more than 125 times the 1x1x.26mm aluminum target.

[Carl-NC]

I would say this is incorrect. High conductivity materials like copper, silver are the most affected by skin effects at high frequencies. Low conductor metals like cupro-nickel, lead, maintain greater skin depths as frequency increases.

I use high & low "conductors" rather loosely to describe the response of the target, not necessarily the metal conductivity. Big thick coins are typically not dominated by skin effect, so that if you e.g. double their thickness the phase response doesn't change much. This is even more true at higher frequencies. If you take it to the extreme, a "superconductor" has a skin depth of zero and will give the exact same phase response (180°) regardless of thickness or frequency.

"Low conductors" like thin aluminum foil are low conductors because of how thin they are, and skin effect dominates their response, not the metal conductivity. Thin hammered silvers become low conductors, and so will just about any other metal. Conversely, if you make a big thick aluminum coin it will respond just like a big thick silver coin.

[Melbeta]

Okay. I have been detecting since the very early 1970s, and I used Garrett BFO machines at first. Then I used TreasureTronics BFO discrimination machines. Then came the Compass IB (TR) machines such as the Yukon machines, Then I used the Compass Judge machiines with discrimination. And I have used the progression of machines from IB/TR, used all kinds of brands, was a dealer for all of the popular machines, and the unpopular machines, have many of them still stored in my basement garage. So I know for a fact from actual experience what works as far as discrimination goes and what works as far as automatic tuning goes. I got so good I could actually see the shape of the target in the ground. And I swear I could hear the hole inside the rings and the inside the pull tabs.

So I look and read with amusement on lots of posted comments.

But now here comes this old comment on SKIN EFFECTS. I have read the term, but not sure I know what you guys are speaking about using the words "skin effects". I havee heard the term alluded to many things. So I want someone or more then one person to explain WHAT a "skin effect" is.... Educate me please!!!!!! The real solid explanation for SKIN EFFECTS please.
Melbeta